fix: replace fetch with node http client for all Overleaf requests

Node fetch can fail on non-Latin1 response headers returned by Overleaf.
This replaces fetch-based Overleaf requests with a shared node http/https
transport to avoid ByteString header failures across list, info, pull, pdf,
output, and related API paths.

Verified locally against the live "You&I" project with list, info, pull,
pdf, and output log.

Author: aloth